1. Field of the Invention
The present invention relates to a pseudo-stereo circuit for converting monophonic audio signals into stereophonic audio signals.
2. Prior Art
FIG. 1 shows an example of a conventional pseudo-stereo circuit. The pseudo-stereo circuit is principally comprised of L-channel phase-shift circuit 100L and R-channel phase-shift circuit 100R for shifting the phase of a monophonic audio signal Min, to generate respective output signals, and a stereo coordination circuit 200 that receives the output signals of the phase-shift circuits 100L and 100R, and produces stereophonic audio signals carried by two channels, i.e., L and R channels.
The L-channel phase-shift circuit 100L includes, for example, three all-pass filters 101L, 102L and 103L that are cascade-connected in this order. Similarly, the R-channel phase-shift circuit 100R includes three all-pass filters 101R, 102R, and 103R similar in structure to the filters 101L, 102L and 103L, that are cascade-connected in this order. Each of these all-pass filters will be described in detail below.
The all-pass filter 101L is comprised of an operational amplifier 301, resistors 302-304, and a capacitor 305, that are connected in the manner as shown in FIG. 1. The resistors 303 and 304 have the same resistance value. Accordingly, the input voltage Vn of the inverting input terminal (xe2x88x92) of the operational amplifier 301 is given by the following expression (1):
Vn=(Min+Vo)/2xe2x80x83xe2x80x83(1)
where Vo is the output voltage of the operational amplifier 301.
On the other hand, the input voltage Vp of the noninverting input terminal (+) of the operational amplifier 301 is given by the following expression (2):
Vp=Min/(1+jxcfx89C1R1)xe2x80x83xe2x80x83(2)
where R1 represents the resistance value of the resistor 302, C1 the capacitance value of the capacitor 305, and xcfx89 the angular frequency of the input monophonic signal Min.
In the circuit arrangement shown in FIG. 1, since the inverting input terminal (xe2x88x92) and noninverting input terminal (+) of the operational amplifier 301 are virtually short-circuited to each other due to negative-feedback operation of the circuit, the input voltage Vp becomes equal to the input voltage Vn, and the following expression (3) is established:
(Min+Vo)/2=Min/(1+jxcfx89C1R1)xe2x80x83xe2x80x83(3)
By transforming the above expression (3), the transfer function of the all-pass filter 101L is obtained as follows:                                                         H              =                              Vo                /                Min                                                                                                        =                                                      (                                          1                      -                                              j                        ⁢                                                  xe2x80x83                                                ⁢                        ω                        ⁢                                                  xe2x80x83                                                ⁢                        C1R1                                                              )                                    /                                      (                                          1                      +                                              j                        ⁢                                                  xe2x80x83                                                ⁢                        ω                        ⁢                                                  xe2x80x83                                                ⁢                        C1R1                                                              )                                                              ⁢                              xe2x80x83                                                                        (        4        )            
Thus, the gain G of the all-pass filter 101L with respect to the input monophonic signal Min is obtained from the above expression (4), and expressed by:                                                         G              =                              "LeftBracketingBar"                H                "RightBracketingBar"                                                                                        =                              "LeftBracketingBar"                                                      (                                          1                      -                                              j                        ⁢                                                  xe2x80x83                                                ⁢                        ω                        ⁢                                                  xe2x80x83                                                ⁢                        C1R1                                                              )                                    /                                      (                                          1                      +                                              j                        ⁢                                                  xe2x80x83                                                ⁢                        ω                        ⁢                                                  xe2x80x83                                                ⁢                        C1R1                                                              )                                                  "RightBracketingBar"                                                                                                        =                                                      "LeftBracketingBar"                                          (                                              1                        -                                                  j                          ⁢                                                      xe2x80x83                                                    ⁢                          ω                          ⁢                                                      xe2x80x83                                                    ⁢                          C1R1                                                                    )                                        "RightBracketingBar"                                    /                                      "LeftBracketingBar"                                          (                                              1                        +                                                  j                          ⁢                                                      xe2x80x83                                                    ⁢                          ω                          ⁢                                                      xe2x80x83                                                    ⁢                          C1R1                                                                    )                                        "RightBracketingBar"                                                              ⁢                              xe2x80x83                                                                                        =              1                                                          (        5        )            
Accordingly, the input monophonic signal Min of any level of frequency passes through the all-pass filter 101L while keeping its amplitude at the same value.
The phase of the input monophonic signal Min is shifted when the signal passes through the all-pass filter 101L. The phase shift amount or phase angle xcex8 is determined depending upon the frequency of the input signal Min, as shown in the following expression (6):                                                         θ              =                              arg                ⁢                                  xe2x80x83                                ⁢                                  (                  H                  )                                                                                                                        =                                                      -                    2                                    ⁢                                      xe2x80x83                                    ⁢                                                            tan                                              -                        1                                                              ⁡                                          (                                              ω                        ⁢                                                  xe2x80x83                                                ⁢                        C1R1                                            )                                                                                  ⁢                              xe2x80x83                                                                        (        6        )            
The all-pass filter 101L has the above described construction and frequency characteristics.
The other all-pass filters 102L and 103L subsequent to the all-pass filter 101L have exactly the same structure as the all-pass filter 101L. As is apparent from the above expression (6), the phase shift amount given by each of the all-pass filters 101L-103L to the input monophonic signal Min varies from 0 to xe2x88x92xcfx80, as the frequency f=xcfx89/2xcfx80 changes. Accordingly, the phase shift amount given by the L-channel phase-shift circuit 100L as a whole to the input signal Min varies from 0 to xe2x88x923xcfx80 as the frequency f of the input signal changes. The phase shift amount xcex8L given by the whole L-channel phase-shift circuit 100L is illustrated in FIG. 2.
The R-channel phase-shift circuit 100 R has basically the same structure as the L-channel phase-shift circuit 100L as explained above, but the resistance value of the resistor 302 and the capacitance value of the capacitor 305 of each of the all-pass filters 101R-103R are different from the values R1 and C1 of the all-pass filters 101L-103L, such that, as shown in FIG. 2, the curve representing the frequency characteristic of the phase shift amount xcex8R of the R-channel phase-shift circuit 100R as a whole is shifted with respect to the curve representing the frequency characteristic of the phase shift amount xcex8L of the L-channel phase-shift circuit 100L in the direction of the X-axis representing the frequency of the input signal.
By appropriately selecting the resistance value of the resistor 302 and the capacitance value of the capacitor 305 in each of the L-channel phase-shift circuit 100L and R-channel phase-shift circuit 100R, a difference (xcex8Lxe2x88x92xcex8R) between the phase shift amounts of these circuits 100L, 100R can be controlled to approximately xcfx80/2 over almost the entire audio frequency band, as shown in FIG. 2. In the circuit shown in FIG. 1, the resistance and capacitance values are suitably selected so that the above requirement is satisfied.
In the circuit arrangement shown in FIG. 1, therefore, the L-channel phase-shift circuit 100L and the R-channel phase-shift circuit 100R output respective audio signals whose phases are shifted with respect to the phase of the input monophonic signal Min and are different from each other by xcfx80/2.
The stereo coordination circuit 200 functions to produce stereophonic audio signals based on the respective output signals of the L-channel phase-shift circuit 100L and R-channel phase-shift circuit 100R as explained above. The stereo coordination circuit 200 is comprised of a subtracter 201, a filter 202, an adder 203 and a subtracter 204. In the thus constructed stereo coordination circuit 200, the subtracter 201 produces a signal corresponding to a difference between the output signals of the L-channel phase-shift circuit 100L and the R-channel phase-shift circuit 100R, and the filter 202 limits the frequency range of the output signal of the subtracter 201. The adder 203 performs addition of the output signal of the filter 202 and the output signal of the L-channel phase-shift circuit 100L. The subtracter 204 performs subtraction between the output signal of the filter 202 and the output signal of the R-channel phase-shift circuit 100R. The adder 203 and the subtracter 204 then generate stereophonic audio signals carried by two channels, or L and R channels, so as to produce sound that affords the listener a sense of the spatial distribution of the sound sources.
When the above-described pseudo-stereo circuit is produced as an integrated circuit or IC, the resulting IC chip has a relatively large area since the circuit requires a large number of constituent components, such as operational amplifiers. Also, the known pseudo-stereo circuit requires six capacitors only in the phase-shift circuits for the L and R channels, and these capacitors are generally required to have large capacitance values. It is, therefore, difficult to form these capacitors on the IC board, in view of the limitation of the chip area, and the capacitors need to be provided outside the IC chip, resulting in an increased number of pins needed to be used in the IC. Under these circumstances, the known pseudo-stereo circuit suffers from undesirably high manufacturing cost.
It is therefore an object of the invention to provide an inexpensive pseudo-stereo circuit having a simplified structure.
To attain the above object, the present invention provides a pseudo-stereo circuit comprising an input terminal that receives an input monophonic signal to be processed, a phase-shift circuit that shifts a phase of the input monophonic signal by a phase shift amount that depends upon a frequency of the monophonic signal, to produce an output signal having a gain with respect to the input monophonic signal which is equal to or larger than a predetermined level over an entire frequency band thereof, and reaches a peak at a frequency at which the phase shift amount of the output signal with respect to the input monophonic signal assumes a value equal or closer to xe2x88x92xcfx80, and a mixing circuit that produces a first mixed signal by mixing a signal obtained by inverting a phase of the output signal of the phase-shift circuit with the input monophonic signal by a first mixing ratio, and produces a second mixed signal obtained by mixing the output signal of the phase-shift circuit with the input monophonic signal by a second mixing ratio, the mixing circuit generating the first mixed signal as a first audio signal carried by one of left and right channels that provide stereophonic audio signals, and generating the second mixed signal as a second audio signal carried by the other of the left and right channels.
Preferably, the phase shift amount of the output signal of the phase-shift circuit with respect to the input monophonic signal changes in a range from 0xcfx80 to xe2x88x922xcfx80 depending upon a frequency of the input monophonic signal.
In a preferred form, the phase-shift circuit comprises first and second phase-shift filters that are cascade-connected, Each of the first and second phase-shift filters comprises an operational amplifier having an inverting input terminal, a noninverting input terminal, and an output terminal, a time-constant circuit formed of a resistance through which an input signal of the filter is transmitted to the noninverting input terminal of the operational amplifier, and a capacitance, an input resistance through which the input signal is transmitted to the inverting input terminal of the operational amplifier, and a feedback resistance interposed between the inverting input terminal and the output terminal of the operational amplifier. A resistance value ratio of the input resistance to the feedback resistance of the first phase-shift filter is set to be greater than 1, and a resistance value ratio of the input resistance to the feedback resistance of the second phase-shift filter is set to be smaller than 1.
Preferably, the first and second phase-shift filters each shift the phase of an input signal thereof by a phase shift amount which changes in e range from 0xcfx80 to xe2x88x922xcfx80 depending upon a frequency of the input monophonic signal, to produce a output signal which is shifted in phase with respect to the input signal.
Also preferably, the first phase-shift filter generates an output signal which has a gain with respect to an input signal thereof, which progressively increases from 1 to a predetermined value as a frequency of the input signal increases, and the second phase-shift filter generates an output signal which has a gain with respect to an input signal thereof, which progressively decreases from 1 to a second predetermined value as a frequency of the input signal increases.
Preferably, the first predetermined value has a reciprocal thereof almost equal to the second predetermined value.
Advantageously, the phase shift amount of the first mixed signal with respect to the input monophonic signal progressively changes in a predetermined direction as a frequency of the monophonic signal changes, and the phase shift amount of the second mixed signal with respect to the input monophonic signal is maintained at an almost constant value irrespective of changes in the frequency of the monophonic signal, the first and second mixing ratios being determined so that frequency characteristics of the gains of the first and second mixed signals with respect to the input monophonic signal are substantially identical to each other over the entire frequency band.
Preferably, the first and second mixed signals each have a gain which reaches a peak at or about a frequency at which a phase difference between the first and second mixed signals is equal to xcfx80.
The above and other objects, features, and advantages of the invention will be become more apparent from the following detailed description taken in conjunction with the accompanying drawings.